Question

Why is the reflecting type telescope preferred over astronomical?

Answer 1

Hint: Astronomical telescope is refracting type and it makes an inverted image of the distant object. It has spherical and chromatic aberrations. We know that the chromatic aberrations occur in the case of lenses and the mirrors are free from the chromatic aberrations.

Complete answer:
As we know, Astronomical telescopes are used to observe and study heavenly bodies. It produces virtual and inverted images. As heavenly bodies like planets and stars are all round, the inverted image does not affect the observation.For observing objects on the earth, we need an erect image. A telescope used for observing objects on earth is called a terrestrial telescope.Reflecting type telescope is preferred over astronomical (refracting type) because it has following advantages:
1. In reflecting type, objective is not lens hence image formed is free from chromatic aberration.
2. Spherical aberration can also be minimized by using parabolic mirrors in the place of concave mirrors.
3. Concave spherical mirrors of large aperture can be easily manufactured.
4. It has high resolving power.
5. As the mirrors weigh less as compared to the lenses, the mechanical support required for the reflecting type telescope is comparatively less.

The following figure shows the image formation by the reflecting type telescope.

Note: Largest reflecting type objective is 234 inch while largest refracting type objective is 40 inch only. To answer this question, students should be familiar with the characteristics of the concave mirrors. The concave mirror is able to form both types of images that are real and virtual images depending on the position of the object from the mirror.